There is a need for anisotropic optical layers or films that demonstrate negative optical retardation dispersion. For example, a quarter wave film made with negative dispersion birefringent materials will be largely achromatic. Devices such as a reflective LCD that utilises such a quarter wave film will have a dark state that is not coloured. Currently such devices have to use two retarder films to achieve this effect. The dispersive power of such a film can be defined in many ways, however one common way is to measure the optical retardation at 450 nm and divide this by the optical retardation measured at 550 nm (R450/R550). If the on-axis retardation of a negative retardation dispersion film at 550 nm is 137.5 nm and the R450/R550 value is 0.82, then such a film will be a largely a quarter wave for all wavelengths of visible light and a liquid crystal display device (LCD) using this film as, for example, a circular polariser would have a substantially black appearance. On the other hand, a film made with an on axis of 137.5 nm which had normal positive dispersion (typically R450/R550=1.13) would only be a quarter wave for one wavelength (550 nm), and an LCD device using this film as, for example, a circular polariser would have a purple appearance. Another way of representing this information is to plot the change in birefringence as a function of wavelength. FIG. 1 shows a typical birefringence against wavelength plot for a polymerised film made from the commercially available reactive mesogen RM257 (Merck KgaA, Darmstadt, Germany). The R450/R550 for this compound is around 1.115.
In an anisotropic optical film formed by rod-shaped, optically anisotropic molecules, the origin of the retardation dispersion is due to the fact that the two refractive indices ne, no, of the anisotropic molecules (wherein ne is the “extraordinary refractive index” in the direction parallel to the long molecular axis, and no is the “ordinary refractive index” in the directions perpendicular to the long molecular axis) are changing with wavelength at different rates, with ne changing more rapidly than no towards the blue end of the visible wavelength spectrum. One way of preparing material with low or negative retardation dispersion is to design molecules with increased no dispersion and decreased ne dispersion. This is schematically shown in FIG. 2. Such an approach has been demonstrated in prior art to give LC's with negative birefringence and positive dispersion as well as compounds with positive birefringence and negative dispersion.
Thus, molecules that can be formed into anisotropic films that demonstrate the property of negative or reverse retardation dispersion have been disclosed in prior art. For example, JP2005-208416 A1 and WO 2006/052001 A1 disclose polymerisable materials based on a “cardo” core group. JP2005-208414 A1 discloses molecules that have covalently bonded discs and rods. JP2005-208415 A1 and JP2002-267838 A1 disclose materials that possess a cross-shape with short high refractive index parts of the molecule crossed with longer lower refractive index parts. WO 2005-085222 A1 discloses molecules that have two lower refractive index parts connected by a higher refractive index bridge part. The bridge is predominantly connected to the rods via a fused five-membered heterocyclic ring. All the above-mentioned documents disclose molecules that not only demonstrate negative dispersion, but also contain at least one polymerisable group and can therefore be polymerised when exposed to either heat or UV irradiation. These materials can be processed either as single materials, or as a mixture to give thin films which under the appropriate conditions can demonstrate uniform anisotropic properties. If photoinitiator is also included in the mixture, the anisotropic properties can be locked in by exposing the film to UV irradiation. This method of preparing optical films is well known.
Another class of materials which is claimed to demonstrate negative birefringence is disclosed in U.S. Pat. No. 6,139,771, which describes compounds generally consisting of two rod-shaped LC parts connected by an acetylenic or bis-acetylenic bridging group. The bridging group is connected to the two rod-shaped parts using a benzene ring.
U.S. Pat. No. 6,203,724 discloses molecules generally consisting of two rod-shaped LC parts connected by highly dispersive bridging groups. The bridging group is connected to the rod-shaped parts via the axial position of a cyclohexane ring. However the document does neither disclose nor suggest to use such compounds for the preparation of birefringent layers having negative optical dispersion.
U.S. Pat. No. 5,567,349 discloses dimers (or H-shaped RM's) wherein the bridging group is connected to the rod shaped part of the molecule via a phenyl ring, however, this document does not report that the molecules demonstrate negative dispersion or negative birefringence.
However, the materials already disclosed in the literature have thermal properties that are not suitable for processing under standard industrial processes, or are not soluble in the solvents commonly used in standard industrial processes or are not compatible with host RM materials commonly used in standard industrial processes, or are too expensive to manufacture.
This invention has the aim of providing improved birefringent layers and materials for their preparation not having the drawbacks of the prior art materials.
Another aim of the invention is to extend the pool of layers and materials having negative dispersion that are available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing birefringent layers and materials as claimed in the present invention.